1. Field of the Invention
The present invention relates to a method and an apparatus for testing the junction strength of an electrode formed on the surface of a substrate, for example, a bump electrode formed on a flip chip IC, to evaluate the junction strength of the same.
2. Description of Related Art
For example, U.S. Pat. No. proposes directly testing the junction strength of soldered bump electrodes by directly pulling the soldered bump electrodes on a flip chip IC with a pointed probe one by one. To be more specific, the above-described U.S. patent discloses a process comprising providing that a probe made of material having good wettability with solder, such as copper, striking the tip end of the probe in or pressing the end of the probe on the soldered bump electrode, heating the probe to melt the bump electrode, and cooling and solidifying to join the probe to the soldered bump electrode, and exerting a pulling force on the probe when the probe is pulled in a junction state. The pulling force is measured by a load cell.
However, in cases in which the above-described apparatus is actually used, it often has large variations in the measurement results and, hence, generally reduces reliability. Through experimental tests and analyses, it was found that those variations are caused by the amount of solder between the probe and the soldered bump electrode.
That is, since in this apparatus the junction between the probe and the bump electrode is made only at the soldered portion for composing the electrode, the amount of solder for joining the probe and bump electrode is relatively reduced. In the state in which the amount of solder for joining the probe to the electrode is small, the junction strength between them is apt to be insufficient and, hence, there may be many cases in which the junction strength between the probe and the bump electrode is smaller than that of the bump electrode to an IC substrate.
Moreover, it was found also that, even if sufficient junction strength between the probe and the bump electrode is provided, when the probe is pulled in the state where the amount of solder for joining the probe to the bump electrode is small, stress concentration is produced at the portion the cross sectional area of which is reduced by the deformation caused by the pulling and the above-described solder is broken before the bump electrode is separated from the substrate. That is, in the state where the small amount of solder is small, as described above, there may be cases where the pulling strength of the soldered portion is smaller than the junction strength of the bump electrode to the IC substrate. In such cases, it is also difficult to accurately measure the junction strength of the soldered bump electrode.
Furthermore, the above apparatus has the drawbacks that an object to be tested is limited to a soldered bump electrode having solder on the surface layer thereof and, hence, it is difficult to evaluate the junction strength of a gold-made bump electrode or a copper-made electrode, which results in the lessened range of applications.